A photomask may be used to transfer a pattern onto a semiconductor wafer. The pattern which is to be transferred onto the wafer may be formed on a material, such as glass or quartz, which is substantially transparent. The material of the photomask may also include thin films of metal or other nontransparent material that prevent light from passing through selected portions of the photomask. In typical photomasks, the nontransparent material is opaque chrome.
Due to limitations imposed by the wavelength of light used to transfer the pattern, resolution degrades at the edges of the patterns of the photomask. Phase-shifting photomasks increase the resolution of patterns by creating phase-shifting regions in the transparent areas of a photomask. Standard phase-shift photomasks generally are formed either by (1) depositing transparent films of appropriate thickness and patterning them over the desired transparent areas using a second level lithography and etch technique, or (2) etching vertical trenches in the substrate. In both of these instances, the phase of light is shifted due to it traveling an additional distance through the transparent phase-shifting area.
The characteristics of the photomask described above generally relate to a hard or strong phase-shifter type. This type of photomask is known as an "alternating aperture" or "Levenson-type" phase-shift mask. These types of masks include transmission areas (light transmitted through the transparent areas) on either side of a patterned opaque feature. One of these transmission areas transmits light, for example, 180.degree. out of phase from the other transmission area, and both sides transmit approximately 100% of the incident radiation. Light diffracted underneath the opaque regions from these phase-shifted regions thus cancels each other, thereby creating a more intense null or "dark area." In particular photomask layouts it is possible that two transmission areas may be adjacent to each other. In certain embodiments these two transmission areas may be at different phase angles.
Fabricating this type of photomask is difficult, as the many steps required to fabricate the mask may cause defects and imperfections. For example, one type of difficulty is the presence of undesired null areas. These null areas may be produced by phase edge transitions in which two phase-shifting layers meet. For example, in FIG. 1, a photomask 5 is shown having both 0.degree. and 180.degree. phase-shift regions. As shown in FIG. 1, a 0.degree. phase pattern 7 may be located next to a 180.degree. phase pattern 9, thereby creating a 0.degree. to 180.degree. phase edge transition 8. As shown in FIG. 1, this transition may cause an undesired null area. This null area may cause an undesired dark feature, known as a "stringer," to be reproduced in a photoresist layer of a wafer being processed using the photomask. This stringer feature may cause a device created on the semiconductor wafer to be inoperative. It is appreciated that stringers or other undesired null features may occur in other configurations and circuit designs.
The present invention presents buffer regions which are disposed between phase transitions of phase-shifting layers of varying phase angles. Further, the present invention performs more than one etch in order to minimize the drawback of defects causing a full 180.degree. phase error which would cause a stringer to be reproduced onto a wafer processed using the photomask.